From (a-ET)PCO2 gradients or differences- Alveolar dead space

Clinical Aspects

Kodali. Bhavani Shankar MD

(a-ET)PCO2 gradients or differences- Alveolar dead space

There are three important applications of (a-ET)CO2 differences.

Monitoring PaCO2

Measurements of PETCO2 constitute a useful non-invasive tool to monitor PaC02 and hence the ventilatory status of patients during anesthesia or in the intensive care unit. In normal individuals, the (a-ET)PC02 may vary from 2-5 mmHg.1-5 The PETCO2 is even more useful if its relationship to PaC02 can be established initially by blood gas analysis. Thereafter, changes in PaC02 may be assumed to occur in parallel with those in PETCO2 thus avoiding repeated arterial puncture provided there are no major hemodynamic changes or respiratory abnormalities that may alter alveolar dead space and hence, (a-ET)PC02. (For details -Physiology section)

Monitoring alveolar dead space

The (a-ET)PCO2 is a measure of alveolar dead space, and changes in alveolar dead space correlate well with changes in (a-ET)PCO2.1 An increase in (a-ET)PCO2 suggests an increase in dead space ventilation. Hence (a-ET)PCO2 is an indirect estimate of V/Q mismatching of the lung.

Monitoring clinical progress of a critical patient

In patients with severe lung disease or hemodynamic instability, the PETCO2 may not be good predictor of PaCO2 because (a-ET)PCO2 gradients vary with the changing V/Q relationship of the lungs, thus making PETCO2 measurements less reliable.6 The emphasis here is on more ABG's until the V/Q mismatch improves and a more constant (a-ET)PCO2 relationship is established. Establishment of constant (a-ET)CO2 implies a good improvement in the V/Q status of the patient.